The present invention relates to an improved Meal-Ready-to-Eat (MRE) Flameless heater product and process for the manufacture thereof, and, more particularly, to an MRE-Flameless product and process which uses a novel heater pad composition to allow lower cost mass production while yielding a superior product.
The manufacture of a MRE-Flameless heater used by, for example, the military is set forth in Military Specification MIL-R-44398A (Jul. 10, 1990). The method of construction as detailed in the military specification is impractical for mass production which, for military use alone, is on the order of about 24 million heaters per month.
The Mil-Spec calls for preparation of an Mg--Fe powder alloy by ball milling of the magnesium and food-grade iron powders and then blending the powdered alloy with an electrolyte, respectively, to form a supercorroding blend. The electrolyte (e.g., salt) is activated by the addition of water, which initiates rapid corrosion of the magnesium particles within the matrix. A surfactant is used to assure that the water wets the alloy surfaces.
In the known heater construction, the supercorroding blend is mixed with plastic powders, such as high density polyethylene powders, and poured into molds. The plastic powders comprise 50% of the mixture mass. The molds are then oven baked to allow the polyethylene material to partially melt and fuse or sinter the mixture together. For military applications, each heater pad has approximate dimensions of 31/2 by 41/2 by 1/8 inches, and contains 20.+-.2 grams of combined heater material and polyethylene filler material (50% by mass mix of each). After sintering, the porous but somewhat rigid material is then packaged into the heater pad covering. The heat pad is then covered with a paperboard cover with dimensions of 51/2 by 41/2 inches, as described in the above-mentioned MIL-R-44398A.
The known method of manufacturing relies on oven sintering for heater pad strength and assembly. This sintering, or partial melting, of the polyethylene material bonds the Mg--Fe particles into a porous structure. There are, however, disadvantages with this approach. In particular, the sintering effectively coats many of the Mg--Fe alloy surfaces thereby rendering them inactive for corrosion and subsequent heat generation. The bottom-side of the sintered structure sinters more completely than the top-side, with the result that this over-sintering causes a polyethylene skin to cover the entire side and reduce the porosity. Slight variations in sintering temperature significantly affect the amount of alloy which is covered by polymeric material and severely affects the total heat energy available. Thus, there is a substantial deviation in the finished product's heater capacity, and product uniformity is a problem. Finally, the sintered material is prone to flaking-off of many small Mg--Fe particles; these particles pass easily through the 0.25-inch diameter holes on the heater pad cover resulting in a less visually attractive product.
A basic problem with present heater pads is the method for packaging them, which method does not lend itself to rapid mass production. It is, therefore, an object of the present invention to provide an alternative packaging method to hold the loose heater materials in place and a product with a specific heater mixture for production of the necessary heat, control of the heating rate and suppression of the flammability of the magnesium alloy, which method does lend itself to rapid mass production.
The foregoing object has been achieved by a unique embodiments of a heater pad construction and currently three associated methods to package this heater pad. Although all three methods appear to perform substantially equivalently, any differences among them involve trade-offs such as, lower cost of tooling or more rapid production capability. Thus, greater options have now been provided than were heretofore available.
The heater pad construction according to the present invention includes a supercorroding Mg--Fe alloy powder, flame-retardant polymer and an electrolyte together with flow and wetting agents dispersed throughout a porous matrix formed from packaging the mixture between polymeric material or cellulose material.
The Mg--Fe alloy is prepared in a known manner by ball milling Mg5Atomic % Fe and then blending the resulting powdered alloy with 3% NaCl (salt), a surfactant (e.g. Triton X102), and optionally, a food-grade acid anhydride or free acid to maintain the proper pH. Food-grade acid can also be applied to the outside of the MRE heater pad, the inside of the outer pouch, or blended with the other ingredients. Flammability control can be achieved by blending a fire retardant into the heater pad materials.
The acid controls the acidity to a value between 4 and 7 pH units. This control is necessary because the rate of reaction of magnesium with water can become too vigorous at low pH and too slow at high pH values. An acid anhydride, such as benzoic anhydride, controls the pH by controlling the rate of hydrolysis of the anhydride to the acid. The acid when formed ionizes in water and lowers the pH. Many organic acids ionize to a limited extent and typically produce solutions with pH values in the 3 to 6 pH range. Thus, control of the formation of the acid by the hydrolysis reaction provides unique control of the heating rate of the MRE pad. The products of the chemical reaction are heat, magnesium hydroxide, and gaseous hydrogen.
Sodium chloride is added to the heater pad mixture at a 3% (0.6 gm) level to provide the needed electrolyte to help cause the water to react with the supercorroding alloy. Magnesium powder is a flammable solid, but when polyethylene treated with known flame retardants is mixed with the powder, it suppresses the flame propagation. A flame suppressant such as a polymer that contains a flame retardant is added to the other ingredients used in the heater pad of the present invention.
In the heater pad described in greater detail below, a supercorroding alloy blend (heater pad mixture) is packaged loose into a heater pad cover or rigid member which provides the strength and rigidity to hold the particles in place without sintering. The heater pad cover has, by way of example only, approximately dimensions of 51/2 by 41/2 by 3/16 inches, and shall contain sufficient heater material mixture to meet the MIL-R-44398A specification. The present invention avoids the use of sintered polymeric filler material to hold the alloy in place. Rather, the present invention utilizes a package to restrain the material and hold the latter uniformly within the desired surface area.
According to one packaging method in accordance with the present invention, the heater pad covering is composed of a rigid molded polymeric back-cover (e.g. polystyrene, polypropylene or polyethylene) molded into sub-packets to hold the heater material. The heater material will be evenly distributed within the sub-packets using an auger filler or some other dispensing mechanism. The top of the heater material will be covered with a flexible, porous cover such as nonwoven polypropylene, melt-blown or spun-blown polypropylene, cellulose-fiber material, tea paper, nonwoven polyethylene, and the like. The cover material is treated with a food-grade surfactant, such as Triton X102, to allow water to easily wet and penetrate the material.
This top cover is thermally or adhesively bonded at each of the contact edges with the bottom rigid cover. One embodiment of a heater pad back-cover is divided into nine sub-packets. Another embodiment of the heater pad is divided into six sub-packets. The more sub-packets, the less material and smaller segments for settling problems The number of sub-packets is, however, practically determined by the quantity of material which can be metered into each sub-packet and the speed associated with this filling operation.
A molded polymeric rigid-back-cover and a porous non-woven polypropylene-web front-cover contains the porous matrix of powders. The rigid-back-cover provides strength and rigidity to the assembly. For a military application, the rigid polymeric back-cover-material can be a light green.
To increase the rate of water migration into each of the sub-packets and to allow a non-wetted surface for escape of the generated hydrogen gas, small holes can be provided by punching or otherwise made through the back of the rigid molded back cover. These holes are smaller than the particle size contained in the sub-packets and yet are much larger than the average pore-size of the porous material on the front side. Because of the polymeric material being a natural non-wetting surface and of the relative large diameter of these holes, water bridging the holes, which could impede the exhaust of hydrogen from the heating reaction (hydrogen is a by product of this corrosion reaction), will not occur.
Instead of using a paperboard cover with nine holes (e.g. 1/4" in diameter) on each side, as in the current, commercially available heater pad, such that particles can fall out into the outer bag, the present invention uses a molded polymeric cover on the back side and a porous wettable cover on the top-side. In this way, a multitude of very small holes is dispersed throughout the top cover. This not only improves the mobility of the water and its ability to wet the Mg--Fe heater mixture, but also will not allow any particles to fall out of the heater pad cover into the outer bag.
According to another aspect of the present invention, the heater pad cover which contains the heater particle mixture is composed of a three layer sandwich. A bottom porous layer of wettable material, such as nonwoven polymeric or cellulose materials such as melt-blown polypropylene, spun-bond polypropylene, cellulose, tea paper, or some other porous wettable polymeric or cellulose material, is bonded to a center layer which has been die-cut to create cavities in the material. The center layer is created of either a cardboard, cellulose, or polymeric material which creates cavities for the heater material and also provide rigidity to the heater. The necessary volume (about 26 ml for the 22 gram military application) is achieved by a combination of the surface area cut-out and the thickness of the center material. The bottom porous layer is first bonded to the center layer to form pockets or cavities to hold the heater material. In the first method described above, the pockets are produced by the molding of the polymeric rigid back cover, whereas in this second method the pockets are formed by bonding a porous material to the bottom of a rigid material which has cut through cavities, for example, by passing the center material through a die-cutter to punch out the necessary hole pattern and then passing the punched-out material along with the bottom porous material through a set of hot rolling wheels to bond the bottom material to the center material.
MRE heater material is then dropped onto the continuously moving strip of pouches. The material is only dropped along the mid-section leaving the edges clear for bonding on the top layer, and the excess is scraped off with a doctor-blade arrangement. Alternately, an auger filler can be used. Finally, the top-layer of porous material is bonded onto the sandwich to complete the assembly. This top material can be either a treated woven or an untreated, woven or nonwoven polymeric or cellulose material which aids in removal of the gas produced. This heater sandwich is then sheared to the proper length.
In this second production method, both the top and bottom of the heater containment material will be a flexible, porous cover, for example, a melt-blown or spun-bond polymeric material or a cellulose fiber material. These covers are thermally or adhesively bonded at all contact edges with the center rigid skeleton. This process has the ability for much faster and more straightforward automation. Because the heater material is doctor-bladed off the surface, a larger number of smaller holes could be die-cut into the center rigid member or skeleton. Also, because the center rigid skeleton is not molded, a cardboard material can, if desired, be used instead of a polymeric material.
In a third production method and configuration of the heater pad, the heater powder is packaged between porous material in an arrangement of separate small bags which are connected together (by way of analogy, an array of tea bags attached together). These bags of heater material do not provide any rigidity and thus, must be packaged with an outer overwrap or cover of cardboard or polymeric material which provides the necessary system rigidity. The outer cover would be adhesively or thermally bonded to the inner porous bags at the bag seams. The outer cover can cover a single side of the porous bags or completely cover the inner bags on both sides. This outer bag is perforated with holes to allow the entrance of water and the escape of hydrogen. Because the outside protective cover, which provides the rigidity, is not molded, a cardboard material can be used, if desired, instead of a polymeric material.
A heater pad with protective polymeric or cardboard cover constructed from one of the methods described above, is sealed within a plastic bag for protecting the heater material from moisture and providing the container to hold the MRE when it is heated as it is now done in a known manner as described, for example, in above-mentioned MIL-R-44398A.